updated for mbed-os 5.5

Revision:
1:57dcf2e26432
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Thu Jul 13 14:46:30 2017 +0000
@@ -0,0 +1,1 @@
+https://github.com/ARMmbed/mbed-os/#8828635da469162cf2854b5287561c663fb96e72